A comparison of three colorimetric methods of ferrous and total reactive iron measurement in freshwaters

Abstract

The measurement of low levels of ferrous and total reactive iron (TRFe) using three common colorimetric methods (bathophenanthroline (BPA), triazine (TPTZ), and ferrozine) in freshwater was evaluated. The BPA method, due to a hexanol extraction step, had unacceptably high limits of detection with small sample volumes. Humic matter also interfered by forming a precipitate. Although ferrozine&s limit of detection was lower, recovery of Fe spikes by TPTZ was much higher in samples with high levels of colored dissolved organic carbon (CDOC) at pH 6 (90 versus 60%) using the standard development time of 10 min because the iron reduction rate is much slower with ferrozine, which requires 1 hr. Recovery of spiked Fe at sample pH 9 was very poor (32 versus 39%). TPTZ with ascorbic acid in samples adjusted to pH 6 is recommended for TRFe analysis. In the absence of a reducing agent and CDOC, TPTZ reagents reduce ferric to ferrous iron (autoreduction), whereas ferrozine has negligible autoreducing properties. CDOC produced significant autoreducing effects using either TPTZ or ferrozine methods, leading to overestimates unless measured immediately upon reagent addition. Hence, immediate measurement with ferrozine is most suitable for ferrous. Ascorbic acid reduces more rapidly than hydroxylamine and does not need to be purified.